We're certain that some of those arrested are innocent. The cops aren't making any distinction between the arrestees, so if they were wrong about some, we have no reason to think they were right about any of them, unless they come up with some compelling evidence to the contrary.

Lurgee

How do we know that you aren't a terrorist. Short answer, we don't. However, in the absence of any firm evidence that you are, we assume you aren't. A bit like we are doing with Tame Iti and the other arrestees. A society that allows armed enforcers to arrest people on suspicion of being terrorists without any evidence that this is so, is not a society I want to live in.

A man is presumed innocent until proven guilty

You don't actually understand what the presumption of innocence means, do you?

It doesn't mean that a person is presumed innocent at every stage of legal proceedings - otherwise, no-one could ever be arrested for anything:

"Well, you're standing over your wife's body with a knife in your hand that exactly matches the 23 stab wounds in her torso, but I must presume your innocent so I'll let you go."

The presumption of innocence means that when the case comes to court, the defendant does not have to prove that he or she didn't commit the crime - the prosecution must show that they did, or at least very likely that it is likely they did.